(NaturalNews) Eating a diet higher in meat and dairy products may significantly boost a man's risk of prostate cancer, researchers have discovered.

"There is a need to identify risk factors for prostate cancer, especially those which can be targeted by therapy and/or lifestyle changes," said lead researcher Andrew Roddam of Oxford University. "Now we know this factor is associated with the disease we can start to examine how diet and lifestyle factors can affect its levels and whether changes could reduce a man's risk."

In a study published in the Annals of Internal Medicine, Roddam and colleagues compiled the results of 12 prior studies on the connection between prostate cancer and insulin-like growth factor 1 (IGF-1), including a total of 3,700 men with prostate cancer and 5,200 without it.

Prior research has shown a strong relationship between meat and dairy consumption and levels of IGF-1, and strict vegetarians are known to have far lower levels of the hormone circulating in their blood.

In the current study, the researchers divided participants into five groups, based on their blood levels of IGF-1. The average participant age was 62 at the time of testing, and the average age of cancer diagnosis was 67.

Men in the group with the highest IGF-1 levels were 40 percent more likely to be diagnosed with prostate cancer than men with the lowest levels. Among men who did develop cancer, higher IGF-1 levels were also associated with a significantly greater rate of tumor spread.

These relationships were independent of age, weight, alcohol consumption or smoking status. There did appear to be a stronger relationship between IGF-1 levels and cancer risk among men who developed less aggressive cancers, but the researchers noted that this could be a statistical fluke.

Roddam noted that a diet high in meat and dairy could probably increase IGF-1 levels by as much as 15 percent.

Researchers have suspected for some time that IGF-1 might be linked to the development of prostate cancer, but this is one of the first studies to supply convincing evidence. The hormone plays a critical role in cell growth and death, regulating child growth and development in particular. In adults, researchers have found that it can sometimes inhibit regular programmed cell death, leading to the abnormal cascade of cell reproduction without death that is characteristic of cancer. Laboratory studies have also found that cells grow more quickly when dosed with IGF-1.

Roddam cautioned that IGF-1 levels cannot currently be used as a tool for cancer screening, however, since all 3,700 men who developed cancer still had IGF-1 levels in the normal range. The prostate specific antigen (PSA) test currently used, he said, is much more effective.

Lesley Walker of Cancer Research UK greeted the study as a welcome addition to the fight to prevent prostate cancer.

"While there are established risk factors associated with prostate cancer of age, family history, and ethnicity, there are no clear data on modifiable risk factors," Walker said. "Research like this is vital to further the work on prevention and treatment of the disease."

Prostate cancer is the second most lethal cancer among men in both the United States and United Kingdom. An estimated 186,320 new cases are diagnosed in the United States each year, leading to approximately 28,660 fatalities annually. It is the most common cancer among men in the United States, which has the highest prostate cancer rates in the world.

Evidence continues to emerge that diet may play a significant role in the development of prostate cancer. A recent study from researchers at Brigham and Women's Hospital and Harvard University, published in The Lancet Oncology, for example, found that men who were overweight or obese at the time of prostate cancer diagnosis were significantly more likely to die from the disease than men with normal body weight. Men with higher insulin levels were also more likely to die than men with normal levels. The combination of high insulin and high body mass index led to a fourfold increase in the risk of dying from the disease. These correlations were found to be independent of any other risk factors, including the stage at which cancer was diagnosed.

Many prostate cancers are slow growing and detected so late in life that there is not much point in treating them, while others can be aggressive and highly lethal. It increasingly appears as if diet may play a role in this distinction.

"Roughly about 10 percent of [diagnosed] men will eventually die of the cancer," researcher Jing Ma said. "The crucial question now facing urologists, oncologists and prostate cancer patients is 'what are the risk factors that can predict the bad cancers?'"